A recently developed type of thermoelectric generator for converting heat energy into electrical energy comprises: (1) first and second reaction zones; (2) a reaction zone separator which (a) separates and essentially completes enclosure of said first and second reaction zones and (b) comprises a cationically-conductive, solid electrolyte that is essentially impermeable to elemental alkali metal and compounds thereof and ionically conductive with respect to cations of said alkali metal; (3) alkali metal within the first reaction zone and in fluid (i.e., liquid and/or vapor) communication with the solid electrolyte; (4) an electrode within the second reaction zone in electrical contact with the solid electrolyte and sufficiently porous to permit alkali metal vapor to pass therethrough; (5) conduction means for electron flow between the alkali metal within the first reaction zone and the electrode; (6) inlet means for introducing the alkali metal into the first reaction zone; and (7) temperature control means adapted to maintain a temperature in the first reaction zone at least 100.degree. in excess of the lowest temperature in the second reaction zone.
In such a device the alkali metal working fluid which is supplied to the first reaction zone is maintained at a pressure P.sub.2 which is greater than P.sub.1, the pressure in the second reaction zone. During operation of such a device the working fluid, e.g., sodium, passes from the first reaction zone to the second and, where the device includes means for pumping the alkali metal back to the first reaction zone the alkali metal completes a closed cycle. Starting in the high pressure region a heat input raises the incoming liquid alkali metal to temperature T.sub.2. The alkali metal then migrates through the solid electrolyte as a cation as a result of the vapor pressure differential (P.sub.2 -P.sub.1) across the membrane. Electrons left behind leave the P.sub.2 region via the negative electrode. On passing through the solid electrolyte membrane, the alkali metal ions are recombined with electrons at the electrode-electrolyte interface, the electrons meanwhile having passed through the electrical load. Neutral alkali metal evaporates from the porous electrode at pressure P.sub.1 and temperature T.sub.2 passing in the gas phase to condense at a temperature T.sub.1 (T.sub.1 &lt; T.sub.2) in the second reaction zone, thus completing the cycle. The process occurring in the electrolyte and at its interface is nearly equivalent to an isothermal expanison of the alkali metal from pressure P.sub.2 to P.sub.1 at temperature T.sub.2. No mechanical parts move, and the work output of the process is electrical only.
As is clear from the above discussion, the porous electrode of the thermoelectric generator serves an important function in the operation of the device. The electrode serves three important functions: (1) it must conduct electrons; (2) it must permit diffusion of alkali metal; and (3) it must make electrical contact with the solid electrolyte. Since the porous electrode performs such critical functions, the selection of the porous electrode material will have a significant effect on the efficiency of the device. In order to maximize the efficiency of the thermoelectric generator it, thus, is the objective of this invention to provide a porous electrode material which will perform the above functions in the most efficient manner.